A standard test frame such as described in U.S. Pat. No. 3,859,848 has an upper end traverse, a lower end traverse below the upper end traverse, and a pair of parallel upright tie rods having upper ends fixed in the upper traverse and lower ends fixed in the lower traverse. A middle traverse between the upper and lower end can be moved vertically by at least two threaded spindles that are vertically fixed in the upper and lower traverses and threaded in the middle traverse. A massive motor can synchronously rotate these spindles synchronously in one direction to raise the middle traverse and in the opposite direction to lower it with great force. Thus a test device carried on the middle spindle can be pressed against a test sample fixed to the upper or lower traverse, or a test sample on the middle traverse can be pressed against a test device on one of the end traverses to make the desired test.
Such a system is fairly complex, with several heavy-duty drives that must be operated perfectly synchronously to avoid canting and jamming of the middle traverse. Accordingly it is known to operate the middle traverse by means of a heavy-duty hydraulic cylinder. This system, however, often has a very short stroke, and once again synchronizing displacement of the outer ends of the middle traverse so it stays perfectly parallel to the upper and lower traverses is fairly difficult.